1. Field of the Invention
The present invention relates to the purification of cumene. More particularly, the invention relates to the removal of the .alpha.-methyl styrene by selective hydrogenation of the side chain in a distillation column reactor. More particularly, the invention relates to a process wherein the .alpha.-methyl styrene is a byproduct from cumene hydroperoxidation phenol process.
2. Related Information
Substantial quantities of .alpha.-methyl styrene (AMS) are produced as a byproduct in the cumene-phenol peroxidation process. Purified cumene is oxidized to form cumene hydroperoxide (CHP) which is then cleaved to phenol and acetone. A small amount of AMS is produced as a byproduct from the decomposition of dimethylbenzyl alcohol which is also formed with CHP in the oxidation step. The AMS can either be recovered as a byproduct or hydrogenated to cumene and recycled to the phenol process. Since the market for AMS is limited, hydrogenation to cumene is the usual option. The typical composition of the cumene/AMS stream is 5 to 80% AMS with trace amounts of methyl benzofuran, mesityl oxide and acetophenone.
Previously, AMS hydrogenations employed a slurry type process using Raney nickel catalyst. The slurry process results in loss of aromatics and has been largely replaced by the fixed bed process. Although the process is effective, it requires two distillation towers and associated equipment, energy in the form of cooling and pressure.
In the first purification column, an AMS topping column, the cumene/AMS mixture which contains some light and heavier hydrocarbons including residual acetone, benzene, ethylbenzene and low and high boiling carbonyls is fractionated to recover the cumene/AMS as side draw which goes to a fixed bed straight pass hydrogenation reactor using palladium on alumina catalyst where AMS is hydrogenated to cumene in a concurrent flow.
Hydrogenation effluent containing mainly cumene with some unreacted AMS and heavies is fractionated in a second column where a bottoms purge is recycled to the topping column, and cumene is removed from the reflux to the oxidation system (phenol process).
U.S. Pat. No. 4,410,755 proposes a process of the purification of crude .alpha.-methyl styrene before hydrogenation by reacting it with acetol in an inert atmosphere in the presence of a noble metal of Group VIII of the periodic table to transfer hydrogen from the acetol to the .alpha.-methyl styrene and decomposing and sweeping away the resultant pyruvic aldehyde.
U.S. Pat. No. 4,822,936 discloses a selective hydrogenation process where a copper catalyst is supported on a gamma alumina. The process described therein is practiced at 0 (atmospheric pressure) to 10 psig and at temperatures of about 5 to 100.degree. C. The preferred pressure is atmospheric (0 psig) and the preferred temperature is below 35.degree. C. Palladium based hydrogenation catalysts have also been used but a large excess of hydrogen has been required resulting in hydrogenation of some of the styrene.
The use of a solid particulate catalyst as part of a distillation structure in a combination distillation column reactor for various reactions is described in U.S. Pat. Nos.: (etherification) 4,232,177; 4,307,254; 4,336,407; 4,504,687; 4,918,243; and 4,978,807; (dimerization) 4,242,530; (hydration) 4,982,022; (dissociation) 4,447,668; and (aromatic alkylation) 4,950,834 and 5,019,669. Additionally U.S. Pat. Nos. 4,302,356 and 4,443,559 disclose catalyst structures which are useful as distillation structures.
It is an advantage of the present invention that the active noble metals may be used to selectively hydrogenate the side chain of the .alpha.-methyl styrene. It is a further advantage that the present process will operate for longer periods without deactivation of the catalyst.